Systems herein generally relate to devices that detect the leading/trailing edge of sheets of media, and more particularly to detection systems that have a vacuum belt with perforations.
Vacuum belts are often used to transport sheets of material, such as sheets of paper, plastic, transparencies, card stock, etc., within printing devices (such as electrostatic printers, inkjet printers, etc.). Such vacuum belts have perforations (which are any form of holes, openings, etc., through the belt), that are open to a vacuum manifold through which air is drawn. The vacuum manifold draws in air through the perforations, which causes the sheets to remain on the top of the belt, even as the belt moves at relatively high speeds. The belt is generally supported between two or more rollers (one or more of which can be driven) and are commonly used to transport sheets from a storage area (e.g., paper tray) or sheet cutting device (when utilizing webs of material) to a printing engine.
In addition, printers improve performance by detecting locations of the leading and trailing edges of the sheets of media. For example, this allows the printing engine to properly align printing on the sheet of media, and avoids applying marking materials (e.g., inks, toners, etc.) to the belt itself. Common sheet edge detection devices include reflective light sensors (e.g., laser sensors) or similar devices; however, such light sensors may not always detect the sheet edges properly, especially when there is little difference between the color, or appearance, of the sheet and the belt because such sensors measure the contrast between the black media transport belt and the white media edge. Problems arise when colored media, such as greys and browns, are used and where the contrast between the media and the belt is not sufficient to properly trigger the sheet edge.